WO2019216608A1

WO2019216608A1 - Method and apparatus for encoding image by using context-based intra-prediction mode information coding

Info

Publication number: WO2019216608A1
Application number: PCT/KR2019/005384
Authority: WO
Inventors: 살레히파메흐디; 김승환
Original assignee: 엘지전자 주식회사
Priority date: 2018-05-07
Filing date: 2019-05-07
Publication date: 2019-11-14

Abstract

A method for decoding an image by a decoding apparatus according to the present invention comprises the steps of: configuring an MPM list of a current block on the basis of a neighboring block of the current block; deriving a context model for an N-th bin for intra-prediction mode information on the basis of an N-th MPM candidate included in the MPM list, wherein N is less than or equal to 3; deriving a value of the intra-prediction mode information by decoding the N-th bin on the basis of the context model; deriving, as an intra-prediction mode of the current block, an MPM candidate having an index value identical to the value of the intra-prediction mode information among MPM candidates of the MPM list; and generating a prediction sample of the current block on the basis of the intra-prediction mode.

Description

Image Coding Method Using Context-based Intra Prediction Mode Information Coding and Its Apparatus

The present invention relates to an image coding technique, and more particularly, to an image decoding method and apparatus using context-based intra prediction mode information coding in an image coding system.

Recently, the demand for high resolution and high quality images such as high definition (HD) images and ultra high definition (UHD) images is increasing in various fields. The higher the resolution and the higher quality of the image data, the more information or bit rate is transmitted than the existing image data. Therefore, the image data can be transmitted by using a medium such as a conventional wired / wireless broadband line or by using a conventional storage medium. In the case of storage, the transmission cost and the storage cost are increased.

Accordingly, a high efficiency image compression technique is required to effectively transmit, store, and reproduce high resolution, high quality image information.

An object of the present invention is to provide a method and apparatus for improving image coding efficiency.

Another object of the present invention is to provide an image decoding method and apparatus for decoding intra prediction mode information on a current block based on a context model.

Another object of the present invention is to provide an image decoding method and apparatus for deriving a context model for coding a bin for intra prediction mode information of a current block based on an MPM candidate of an MPM list for the current block.

Another object of the present invention is to provide a context for coding a bin for intra prediction mode information of a current block based on the intra prediction mode indicated by the MPM candidate of the MPM list for the current block and / or the order in the MPM list of the MPM candidate. An image decoding method and apparatus for deriving a model are provided.

According to an embodiment of the present invention, there is provided an image decoding method performed by a decoding apparatus. The method includes constructing a Most Probable Mode (MPM) list of the current block based on neighboring blocks of the current block, and assigning an Nth bin to intra prediction mode information based on an Nth MPM candidate included in the MPM list. Deriving a context model for the context model, wherein N is 3 or less, decoding the N th bin based on the context model, and deriving a value of the intra prediction mode information; MPM candidates of the MPM list Deriving an MPM candidate having an index value equal to that of the intra prediction mode information as an intra prediction mode for the current block, and generating a prediction sample for the current block based on the intra prediction mode. Characterized in that.

According to another embodiment of the present invention, a decoding apparatus for performing image decoding is provided. The decoding apparatus derives a context model for the Nth bin for the intra prediction mode information based on the Nth MPM candidate included in the MPM list, wherein N is 3 or less and based on the context model. An entropy decoding unit configured to decode the N-th bin to derive the value of the intra prediction mode information, and construct the MPM list of the current block based on a neighboring block of the current block, wherein the MPM candidates of the MPM list And a prediction unit for deriving an MPM candidate having an index value equal to that of intra prediction mode information as an intra prediction mode for the current block and generating a prediction sample for the current block based on the intra prediction mode. do.

According to another embodiment of the present invention, a video encoding method performed by an encoding apparatus is provided. The method comprises constructing a Most Probable Mode (MPM) list of the current block based on neighboring blocks of the current block, selecting one of the MPM candidates of the MPM list, and selecting the selected MPM candidate intra for the current block. Determining a prediction mode, generating a prediction sample for the current block based on the intra prediction mode, generating at least one bin for intra prediction mode information indicating the selected MPM candidate, and the MPM list Deriving a context model for an Nth bin for the intra prediction mode information based on an Nth MPM candidate included in N, wherein N is 3 or less, and the intra prediction mode based on the context model. Encoding said N-th bin for information.

According to another embodiment of the present invention, a video encoding apparatus is provided. The encoding apparatus constructs a Most Probable Mode (MPM) list of the current block based on neighboring blocks of the current block, selects one of the MPM candidates of the MPM list, and selects the selected MPM candidate intra for the current block. A prediction unit configured to determine a prediction mode, generate a prediction sample for the current block based on the intra prediction mode, and generate at least one bin for intra prediction mode information indicating the selected MPM candidate, and generate the MPM list A context model is derived for an Nth bin for the intra prediction mode information based on an Nth MPM candidate included in the N, wherein N is 3 or less, and the intra prediction mode is based on the context model. And an entropy encoding unit for encoding the Nth bin for the information.

According to the present invention, a context model for coding a bin for intra prediction mode information of the current block may be derived in consideration of the MPM candidate of the MPM list of the current block, thereby indicating the intra prediction mode of the current block. Can reduce the amount of bits and improve the overall coding efficiency.

According to the present invention, a context model for coding a bin for intra prediction mode information of the current block may be derived in consideration of the intra prediction mode indicated by the MPM candidate of the MPM list of the current block and the order in the MPM list of the MPM candidate. Through this, it is possible to reduce the amount of bits for indicating the intra prediction mode of the current block and to improve the overall coding efficiency.

1 is a diagram schematically illustrating a configuration of a video encoding apparatus to which the present invention may be applied.

2 is a diagram schematically illustrating a configuration of a video decoding apparatus to which the present invention may be applied.

3 exemplarily shows intra directional modes of 65 prediction directions.

4 exemplarily illustrates neighboring blocks of the current block.

5 schematically illustrates an image encoding method by an encoding apparatus according to the present invention.

6 schematically illustrates an encoding apparatus for performing an image encoding method according to the present invention.

7 schematically illustrates an image decoding method by a decoding apparatus according to the present invention.

8 schematically illustrates a decoding apparatus for performing an image decoding method according to the present invention.

As the present invention allows for various changes and numerous embodiments, particular embodiments will be illustrated in the drawings and described in detail in the written description. However, this is not intended to limit the invention to the specific embodiments. The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the spirit of the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. The terms "comprise" or "having" in this specification are intended to indicate that there is a feature, number, step, operation, component, part, or combination thereof described in the specification, and one or more other features It is to be understood that the numbers, steps, operations, components, parts or figures do not exclude in advance the presence or possibility of adding them.

On the other hand, each configuration in the drawings described in the present invention are shown independently for the convenience of description of the different characteristic functions, it does not mean that each configuration is implemented by separate hardware or separate software. For example, two or more of each configuration may be combined to form one configuration, or one configuration may be divided into a plurality of configurations. Embodiments in which each configuration is integrated and / or separated are also included in the scope of the present invention without departing from the spirit of the present invention.

Hereinafter, with reference to the accompanying drawings, it will be described in detail a preferred embodiment of the present invention. Hereinafter, the same reference numerals are used for the same components in the drawings, and redundant description of the same components is omitted.

On the other hand, the present invention relates to video / image coding. For example, the method / embodiment disclosed herein may be applied to the method disclosed in the versatile video coding (VVC) standard or the next generation video / image coding standard.

In the present specification, a picture generally refers to a unit representing one image in a specific time zone, and a slice / tile is a unit constituting a part of a picture in coding. The slice / tile may comprise one or more coding tree units (CTUs). One picture may consist of a plurality of slices / tiles, and one tile group may include one or more tiles.

A pixel or a pel may refer to a minimum unit constituting one picture (or image). Also, 'sample' may be used as a term corresponding to a pixel. A sample may generally represent a pixel or a value of a pixel, and may only represent pixel / pixel values of the luma component, or only pixel / pixel values of the chroma component.

A unit represents the basic unit of image processing. The unit may include at least one of a specific region of the picture and information related to the region. The unit may be used interchangeably with terms such as block or area in some cases. In a general case, an M × N block may represent a set of samples or transform coefficients composed of M columns and N rows.

Referring to FIG. 1, the video encoding apparatus 100 may include a picture splitter 105, a predictor 110, a residual processor 120, an entropy encoder 130, an adder 140, and a filter 150. ) And memory 160. The residual processing unit 120 may include a subtraction unit 121, a conversion unit 122, a quantization unit 123, a reordering unit 124, an inverse quantization unit 125, and an inverse conversion unit 126.

The picture divider 105 may divide the input picture into at least one processing unit.

As an example, the processing unit may be called a coding unit (CU). In this case, the coding unit may be recursively split from the largest coding unit (LCU) according to a quad-tree binary-tree (QTBT) structure. For example, one coding unit may be divided into a plurality of coding units of a deeper depth based on a quad tree structure and / or a binary tree structure. In this case, for example, the quad tree structure may be applied first and the binary tree structure may be applied later. Alternatively, the binary tree structure may be applied first. The coding procedure according to the present invention may be performed based on the final coding unit that is no longer split. In this case, the maximum coding unit may be used as the final coding unit immediately based on coding efficiency according to the image characteristic, or if necessary, the coding unit is recursively divided into coding units of lower depths and optimized. A coding unit of size may be used as the final coding unit. Here, the coding procedure may include a procedure of prediction, transform, and reconstruction, which will be described later.

As another example, the processing unit may include a coding unit (CU) prediction unit (PU) or a transform unit (TU). The coding unit may be split from the largest coding unit (LCU) into coding units of deeper depths along the quad tree structure. In this case, the maximum coding unit may be used as the final coding unit immediately based on coding efficiency according to the image characteristic, or if necessary, the coding unit is recursively divided into coding units of lower depths and optimized. A coding unit of size may be used as the final coding unit. If a smallest coding unit (SCU) is set, the coding unit may not be split into smaller coding units than the minimum coding unit. Here, the final coding unit refers to a coding unit that is the basis of partitioning or partitioning into a prediction unit or a transform unit. The prediction unit is a unit partitioning from the coding unit and may be a unit of sample prediction. In this case, the prediction unit may be divided into sub blocks. The transform unit may be divided along the quad tree structure from the coding unit, and may be a unit for deriving a transform coefficient and / or a unit for deriving a residual signal from the transform coefficient. Hereinafter, a coding unit may be called a coding block (CB), a prediction unit is a prediction block (PB), and a transform unit may be called a transform block (TB). A prediction block or prediction unit may mean a specific area in the form of a block within a picture, and may include an array of prediction samples. In addition, a transform block or a transform unit may mean a specific area in a block form within a picture, and may include an array of transform coefficients or residual samples.

The prediction unit 110 may perform a prediction on a block to be processed (hereinafter, referred to as a current block) and generate a predicted block including prediction samples of the current block. The unit of prediction performed by the prediction unit 110 may be a coding block, a transform block, or a prediction block.

The prediction unit 110 may determine whether intra prediction or inter prediction is applied to the current block. As an example, the prediction unit 110 may determine whether intra prediction or inter prediction is applied on a CU basis.

In the case of intra prediction, the prediction unit 110 may derive a prediction sample for the current block based on reference samples outside the current block in the picture to which the current block belongs (hereinafter, referred to as the current picture). In this case, the prediction unit 110 may (i) derive the prediction sample based on the average or interpolation of neighboring reference samples of the current block, and (ii) the neighbor reference of the current block. The prediction sample may be derived based on a reference sample present in a specific (prediction) direction with respect to the prediction sample among the samples. In case of (i), it may be called non-directional mode or non-angle mode, and in case of (ii), it may be called directional mode or angular mode. In intra prediction, the prediction mode may have, for example, 33 directional prediction modes and at least two non-directional modes. The non-directional mode may include a DC prediction mode and a planner mode (Planar mode). The prediction unit 110 may determine the prediction mode applied to the current block by using the prediction mode applied to the neighboring block.

In the case of inter prediction, the prediction unit 110 may derive the prediction sample for the current block based on the sample specified by the motion vector on the reference picture. The prediction unit 110 may apply one of a skip mode, a merge mode, and a motion vector prediction (MVP) mode to derive a prediction sample for the current block. In the skip mode and the merge mode, the prediction unit 110 may use the motion information of the neighboring block as the motion information of the current block. In the skip mode, unlike the merge mode, the difference (residual) between the prediction sample and the original sample is not transmitted. In the MVP mode, the motion vector of the current block may be derived using the motion vector of the neighboring block as a motion vector predictor.

In the case of inter prediction, the neighboring block may include a spatial neighboring block existing in the current picture and a temporal neighboring block present in the reference picture. A reference picture including the temporal neighboring block may be called a collocated picture (colPic). The motion information may include a motion vector and a reference picture index. Information such as prediction mode information and motion information may be encoded (entropy) and output in the form of a bitstream.

When the motion information of the temporal neighboring block is used in the skip mode and the merge mode, the highest picture on the reference picture list may be used as the reference picture. Reference pictures included in a reference picture list may be sorted based on a difference in a picture order count (POC) between a current picture and a corresponding reference picture. The POC corresponds to the display order of the pictures and may be distinguished from the coding order.

The subtraction unit 121 generates a residual sample which is a difference between the original sample and the prediction sample. When the skip mode is applied, residual samples may not be generated as described above.

The transform unit 122 generates transform coefficients by transforming the residual sample in units of transform blocks. The transform unit 122 may perform the transform according to the size of the transform block and the prediction mode applied to the coding block or the prediction block that spatially overlaps the transform block. For example, if intra prediction is applied to the coding block or the prediction block that overlaps the transform block, and the transform block is a 4 × 4 residual array, the residual sample is configured to perform a discrete sine transform (DST) transform kernel. The residual sample may be transformed using a discrete cosine transform (DCT) transform kernel.

The quantization unit 123 may quantize the transform coefficients to generate quantized transform coefficients.

The reordering unit 124 rearranges the quantized transform coefficients. The reordering unit 124 may reorder the quantized transform coefficients in the form of a block into a one-dimensional vector form through a coefficient scanning method. Although the reordering unit 124 has been described in a separate configuration, the reordering unit 124 may be part of the quantization unit 123.

The entropy encoding unit 130 may perform entropy encoding on the quantized transform coefficients. Entropy encoding may include, for example, encoding methods such as exponential Golomb, context-adaptive variable length coding (CAVLC), context-adaptive binary arithmetic coding (CABAC), and the like. The entropy encoding unit 130 may encode information necessary for video reconstruction other than the quantized transform coefficient (for example, a value of a syntax element) together or separately. Entropy encoded information may be transmitted or stored in units of network abstraction layer (NAL) units in the form of bitstreams.

The inverse quantization unit 125 inversely quantizes the quantized values (quantized transform coefficients) in the quantization unit 123, and the inverse transformer 126 inversely transforms the inverse quantized values in the inverse quantization unit 125 to generate a residual sample. Create

The adder 140 reconstructs the picture by combining the residual sample and the predictive sample. The residual sample and the predictive sample may be added in units of blocks to generate a reconstructed block. Although the adder 140 has been described in a separate configuration, the adder 140 may be part of the predictor 110. On the other hand, the adder 140 may be called a restoration unit or a restoration block generation unit.

The filter unit 150 may apply a deblocking filter and / or a sample adaptive offset to the reconstructed picture. Through deblocking filtering and / or sample adaptive offset, the artifacts of the block boundaries in the reconstructed picture or the distortion in the quantization process can be corrected. The sample adaptive offset may be applied on a sample basis and may be applied after the process of deblocking filtering is completed. The filter unit 150 may apply an adaptive loop filter (ALF) to the reconstructed picture. ALF may be applied to the reconstructed picture after the deblocking filter and / or sample adaptive offset is applied.

The memory 160 may store reconstructed pictures (decoded pictures) or information necessary for encoding / decoding. Here, the reconstructed picture may be a reconstructed picture after the filtering process is completed by the filter unit 150. The stored reconstructed picture may be used as a reference picture for (inter) prediction of another picture. For example, the memory 160 may store (reference) pictures used for inter prediction. In this case, pictures used for inter prediction may be designated by a reference picture set or a reference picture list.

Referring to FIG. 2, the video decoding apparatus 200 may include an entropy decoding unit 210, a residual processor 220, a predictor 230, an adder 240, a filter 250, and a memory 260. It may include. Here, the residual processor 220 may include a rearrangement unit 221, an inverse quantization unit 222, and an inverse transform unit 223.

When a bitstream including video information is input, the video decoding apparatus 200 may restore video in response to a process in which video information is processed in the video encoding apparatus.

For example, the video decoding apparatus 200 may perform video decoding using a processing unit applied in the video encoding apparatus. Thus, the processing unit block of video decoding may be, for example, a coding unit, and in another example, a coding unit, a prediction unit, or a transform unit. The coding unit may be split along the quad tree structure and / or binary tree structure from the largest coding unit.

The prediction unit and the transform unit may be further used in some cases, in which case the prediction block is a block derived or partitioned from the coding unit and may be a unit of sample prediction. At this point, the prediction unit may be divided into subblocks. The transform unit may be divided along the quad tree structure from the coding unit, and may be a unit for deriving a transform coefficient or a unit for deriving a residual signal from the transform coefficient.

The entropy decoding unit 210 may parse the bitstream and output information necessary for video reconstruction or picture reconstruction. For example, the entropy decoding unit 210 decodes information in a bitstream based on a coding method such as exponential Golomb coding, CAVLC, or CABAC, quantized values of syntax elements required for video reconstruction, and transform coefficients for residuals. Can be output.

More specifically, the CABAC entropy decoding method receives a bin corresponding to each syntax element in a bitstream, and decodes syntax element information and decoding information of neighboring and decoding target blocks or information of symbols / bins decoded in a previous step. The context model may be determined using the context model, the probability of occurrence of a bin may be predicted according to the determined context model, and arithmetic decoding of the bin may be performed to generate a symbol corresponding to the value of each syntax element. have. In this case, the CABAC entropy decoding method may update the context model by using the information of the decoded symbol / bin for the context model of the next symbol / bean after determining the context model.

The information related to the prediction among the information decoded by the entropy decoding unit 210 is provided to the prediction unit 230, and the residual value on which the entropy decoding has been performed by the entropy decoding unit 210, that is, the quantized transform coefficient, is used as a reordering unit ( 221 may be input.

The reordering unit 221 may rearrange the quantized transform coefficients in a two-dimensional block form. The reordering unit 221 may perform reordering in response to coefficient scanning performed by the encoding apparatus. Here, the rearrangement unit 221 has been described in a separate configuration, but the rearrangement unit 221 may be part of the inverse quantization unit 222.

The inverse quantization unit 222 may dequantize the quantized transform coefficients based on the (inverse) quantization parameter and output the transform coefficients. In this case, information for deriving a quantization parameter may be signaled from the encoding apparatus.

The inverse transform unit 223 may inversely transform transform coefficients to derive residual samples.

The prediction unit 230 may perform prediction on the current block and generate a predicted block including prediction samples for the current block. The unit of prediction performed by the prediction unit 230 may be a coding block, a transform block, or a prediction block.

The prediction unit 230 may determine whether to apply intra prediction or inter prediction based on the information about the prediction. In this case, a unit for determining which of intra prediction and inter prediction is to be applied and a unit for generating a prediction sample may be different. In addition, the unit for generating a prediction sample in inter prediction and intra prediction may also be different. For example, whether to apply inter prediction or intra prediction may be determined in units of CUs. In addition, for example, in inter prediction, a prediction mode may be determined and a prediction sample may be generated in PU units, and in intra prediction, a prediction mode may be determined in PU units and a prediction sample may be generated in TU units.

In the case of intra prediction, the prediction unit 230 may derive the prediction sample for the current block based on the neighbor reference samples in the current picture. The prediction unit 230 may derive the prediction sample for the current block by applying the directional mode or the non-directional mode based on the neighbor reference samples of the current block. In this case, the prediction mode to be applied to the current block may be determined using the intra prediction mode of the neighboring block.

In the case of inter prediction, the prediction unit 230 may derive the prediction sample for the current block based on the sample specified on the reference picture by the motion vector on the reference picture. The prediction unit 230 may apply any one of a skip mode, a merge mode, and an MVP mode to derive a prediction sample for the current block. In this case, motion information required for inter prediction of the current block provided by the video encoding apparatus, for example, information about a motion vector, a reference picture index, and the like may be obtained or derived based on the prediction information.

In the skip mode and the merge mode, the motion information of the neighboring block may be used as the motion information of the current block. In this case, the neighboring block may include a spatial neighboring block and a temporal neighboring block.

The prediction unit 230 may construct a merge candidate list using motion information of available neighboring blocks, and may use information indicated by the merge index on the merge candidate list as a motion vector of the current block. The merge index may be signaled from the encoding device. The motion information may include a motion vector and a reference picture. When the motion information of the temporal neighboring block is used in the skip mode and the merge mode, the highest picture on the reference picture list may be used as the reference picture.

In the skip mode, unlike the merge mode, the difference (residual) between the prediction sample and the original sample is not transmitted.

In the MVP mode, the motion vector of the current block may be derived using the motion vector of the neighboring block as a motion vector predictor. In this case, the neighboring block may include a spatial neighboring block and a temporal neighboring block.

For example, when the merge mode is applied, a merge candidate list may be generated by using a motion vector of a reconstructed spatial neighboring block and / or a motion vector corresponding to a Col block, which is a temporal neighboring block. In the merge mode, the motion vector of the candidate block selected from the merge candidate list is used as the motion vector of the current block. The information about the prediction may include a merge index indicating a candidate block having an optimal motion vector selected from candidate blocks included in the merge candidate list. In this case, the prediction unit 230 may derive the motion vector of the current block by using the merge index.

As another example, when the Motion Vector Prediction (MVP) mode is applied, a motion vector predictor candidate list may be generated using a motion vector of a reconstructed spatial neighboring block and / or a motion vector corresponding to a Col block, which is a temporal neighboring block. Can be. That is, the motion vector of the reconstructed spatial neighboring block and / or the Col vector, which is a temporal neighboring block, may be used as a motion vector candidate. The prediction information may include a prediction motion vector index indicating an optimal motion vector selected from the motion vector candidates included in the list. In this case, the prediction unit 230 may select the predicted motion vector of the current block from the motion vector candidates included in the motion vector candidate list using the motion vector index. The prediction unit of the encoding apparatus may obtain a motion vector difference (MVD) between the motion vector of the current block and the motion vector predictor, and may encode the output vector in a bitstream form. That is, MVD may be obtained by subtracting the motion vector predictor from the motion vector of the current block. In this case, the prediction unit 230 may obtain a motion vector difference included in the information about the prediction, and derive the motion vector of the current block by adding the motion vector difference and the motion vector predictor. The prediction unit may also obtain or derive a reference picture index or the like indicating a reference picture from the information about the prediction.

The adder 240 may reconstruct the current block or the current picture by adding the residual sample and the predictive sample. The adder 240 may reconstruct the current picture by adding the residual sample and the predictive sample in block units. Since the residual is not transmitted when the skip mode is applied, the prediction sample may be a reconstruction sample. Although the adder 240 has been described in a separate configuration, the adder 240 may be part of the predictor 230. On the other hand, the adder 240 may be called a restoration unit or a restoration block generation unit.

The filter unit 250 may apply the deblocking filtering sample adaptive offset, and / or ALF to the reconstructed picture. In this case, the sample adaptive offset may be applied in units of samples and may be applied after deblocking filtering. ALF may be applied after deblocking filtering and / or sample adaptive offset.

The memory 260 may store reconstructed pictures (decoded pictures) or information necessary for decoding. Here, the reconstructed picture may be a reconstructed picture after the filtering process is completed by the filter unit 250. For example, the memory 260 may store pictures used for inter prediction. In this case, pictures used for inter prediction may be designated by a reference picture set or a reference picture list. The reconstructed picture can be used as a reference picture for another picture. In addition, the memory 260 may output the reconstructed picture in an output order.

On the other hand, when intra prediction is applied to the current block, as described above, the encoding device / decoding device may derive an intra prediction mode for the current block, and predict the sample of the current block based on the intra prediction mode. Can be derived. That is, the encoding device / decoding device may derive the prediction sample of the current block by applying the directional mode or the non-directional mode based on the peripheral reference samples of the current block.

For reference, for example, the intra prediction mode includes two non-directional or non-angular intra prediction modes and 65 directional or angular intra prediction modes. Can include them. The non-directional intra prediction modes may include a planar intra prediction mode of 0 and a DC intra prediction mode of 1, and the directional intra prediction modes may include 65 intra prediction modes of 2 to 66. . However, as an example, the present invention may be applied to a case where the number of intra prediction modes is different. In some cases, the intra prediction mode 67 may further be used, and the intra prediction mode 67 may represent a linear model (LM) mode.

3 exemplarily shows intra directional modes of 65 prediction directions.

Referring to FIG. 3, an intra prediction mode having horizontal directionality and an intra prediction mode having vertical directionality may be distinguished from the intra prediction mode 34 having a left upward diagonal prediction direction. H and V in FIG. 3 mean horizontal directionality and vertical directionality, respectively, and numbers of -32 to 32 represent a displacement of 1/32 on a sample grid position. For example, the intra prediction modes 2 to 34 have a horizontal direction, and the intra prediction modes 35 to 66 have a vertical direction. Intra prediction mode 18 and intra prediction mode 50 indicate a horizontal intra prediction mode and a vertical intra prediction mode, respectively, and an intra prediction mode 2 indicates a left downward diagonal intra prediction mode, The 34th intra prediction mode may be referred to as a left upward diagonal intra prediction mode, and the 66th intra prediction mode may be referred to as a right upward diagonal intra prediction mode.

Meanwhile, when the intra prediction is applied to the current block, an intra prediction mode applied to the current block may be derived based on the intra prediction mode of the neighboring block of the current block. For example, the decoding apparatus may derive the most probable mode (MPM) list based on the intra prediction mode and additional candidate modes of the neighboring block (eg, the left neighboring block and / or the upper neighboring block) of the current block. One of the MPM candidates in the derived MPM list may be selected based on the received MPM index, or one of the remaining intra prediction modes not included in the MPM candidates is based on intra prediction mode information. Can be selected. The MPM list may be referred to as an intra prediction mode candidate list or may be referred to as candModeList.

Here, for example, the MPM list may include three MPM candidates, five candidates, or six MPM candidates. For example, the MPM list may include candidates derived based on an intra prediction mode, a derived intra prediction mode, and / or a default intra prediction mode of a neighboring block. The encoding device / decoding device may search the neighboring blocks of the current block in a specific order and derive the intra prediction mode of the neighboring block as the MPM candidate in the derived order. For example, the neighboring blocks may include a left neighboring block, an upper neighboring block, a lower left neighboring block, a right upper neighboring block, and an upper left neighboring block, and the encoding device / decoding device is an intra prediction mode of the left neighboring block. , Intra prediction mode of the upper neighboring block, planner intra prediction mode, DC intra prediction mode, intra prediction mode of the lower left neighboring block, intra prediction mode of the upper right neighboring block, and intra prediction mode of the upper left neighboring block The MPM candidate may be derived and the MPM list of the current block may be constructed. Meanwhile, if six MPM candidates are not derived after the search, an MPM candidate may be derived based on an intra prediction mode derived as the MPM candidate. For example, when the intra prediction mode derived as the MPM candidate is N intra prediction mode, the encoding device / decoding device selects the N + 1 intra prediction mode and / or the N-1 intra prediction mode from the current block. Can be derived as an MPM candidate. Meanwhile, the peripheral blocks may be as described below.

4 exemplarily illustrates neighboring blocks of the current block. Referring to FIG. 4, when the size of the current block is WxH and the x component of the top-left sample position of the current block is 0 and the y component is 0, the left neighboring block is (-1, H-1 ) A block containing a sample of coordinates, wherein the upper peripheral block is a block containing a sample of (W-1, -1) coordinates, and the right upper peripheral block includes a sample of (W, -1) coordinates The lower left peripheral block may be a block including samples of (-1, H) coordinates, and the upper left peripheral block may be a block including samples of (-1, -1) coordinates.

Meanwhile, for example, whether an intra prediction mode applied to the current block is included in the MPM candidates or the remaining intra prediction modes may be derived based on an MPM flag. Specifically, when the value of the MPM flag is 1, the MPM flag may indicate that the intra prediction mode of the current block is included in MPM candidates (MPM list), and when the value of the MPM flag is 0, the MPM The flag may indicate that the intra prediction mode for the current block is included in the remaining intra prediction modes rather than included in MPM candidates (MPM list). The MPM index may be signaled in the form of an mpm_idx or intra_luma_mpm_idx syntax element, and the remaining intra prediction mode information may be signaled in the form of a rem_intra_luma_pred_mode or intra_luma_mpm_remainder syntax element. In addition, the MPM flag may be signaled in the form of an intra_luma_mpm_flag syntax element, and when the MPM flag is not signaled, the value of the MPM flag may be regarded as 1. For example, the remaining intra prediction mode information may index one of the remaining intra prediction modes not included in the MPM candidates among all the intra prediction modes in order of prediction mode number. The intra prediction mode may be an intra prediction mode for a luma component (sample). Hereinafter, the intra prediction mode information may include at least one of the MPM flag, the MPM index, and the remaining intra prediction mode information. In addition, the MPM list may be called in various terms such as an MPM candidate list and a candModeList.

Meanwhile, the encoding apparatus may derive an MPM list for the current block based on the intra prediction mode and additional candidate modes of the neighboring block of the current block, determine the intra prediction mode of the current block, and determine the current block. Intra prediction mode information for may be encoded and stored and / or transmitted.

In general, as described above, the current block and the neighboring block to be coded may have similar image characteristics, and thus, the current block and the neighboring block have a high probability of having the same or similar intra prediction mode. The MPM list of the current block may be configured to derive an intra prediction mode applied to the block, and one of the MPM candidates in the configured MPM list is based on received intra prediction mode information (eg, MPM index). Can be derived. The intra prediction mode information is encoded / decoded through context-based adaptive binary arithmetic coding (CABAC) or context-based adaptive variable length coding (CAVLC). Can be.

Here, for example, the CABAC decoding method, as described above, receives a bin corresponding to the target syntax element in the bitstream, decodes the information on the target syntax element and decoding information of the neighboring block and the current block or the previous step. A context model for the target syntax element is determined based on the information of the symbol / bin, and the arithmetic decoding of the bin is performed by predicting the occurrence probability of the bin according to the determined context model. In this case, a method of generating a symbol corresponding to a value of each syntax element may be indicated. Here, the context model represents the statistical characteristics of the bin, and the context model may have a probability state index and an MPS value. In addition, there may be a plurality of context models for the bin, and the probability state index and the most probable symbol (MPS) value may be different for each context model. In this case, the MPS may represent a value of a bin appearing more in all syntax elements, and the MPS probability state index may represent a probability of the MPS appearing.

As an example, the intra prediction mode information (eg, MPM index) may be encoded / decoded through CABAC. In detail, the MPM list may include N MPM candidates, and an MPM index indicating a selected value of 0 to N−1 may be encoded / decoded. For example, when the MPM list includes six MPM candidates, an MPM index indicating a selected value of 0 to 5 may be encoded / decoded. First, the MPM index can be binarized using Trunked Unary code. The value of the MPM index binarized through the truncated binary code may be represented as in the following table.

Referring to Table 1, the MPM index may be derived as a binary value of 1 to 5 bins according to a value represented. Here, for example, three bins in the preceding order of the bins of the MPM index, that is, bin 0, bin 1, and bin 2 may be coded based on a context model, and bin 4 and bin 5 may be bypassed. bypass) coding may be applied. The bypass coding may represent a method of coding by applying a uniform probability distribution (for example, 50:50) instead of applying a context model having a specific probability distribution. Meanwhile, according to an embodiment of the present invention described below, a method of deriving the context model for the intra prediction mode information based on the MPM candidate of the MPM list for the current block is provided. The context model for the bin 0 may be derived based on the MPM candidate for, the context model for the bin 1 may be derived based on the MPM candidate for bin 1 among the MPM candidates in the MPM list. A context model for the bin 2 may be derived based on the MPM candidate for the bin 2 among the MPM candidates in the MPM list. For example, a context model for the bin 0 may be derived based on the first order MPM candidates among the MPM candidates in the MPM list, and based on the second order MPM candidates among the MPM candidates in the MPM list, the bin 1 may be derived. A context model for may be derived, and a context model for the bin 2 may be derived based on the third order MPM candidates among the MPM candidates in the MPM list.

As described above, when the intra prediction mode information is coded based on a context model, in order to improve coding efficiency of the intra prediction mode information, a more sophisticated context model selection for the intra prediction mode information is performed. ) Is required. Accordingly, the present invention proposes a method of setting a different context model according to the intra prediction mode and deriving the context model for the intra prediction mode information based on the MPM candidate of the MPM list for the current block. The statistical characteristics applied to the prediction may differ depending on the intra prediction mode, so setting different context models according to the intra prediction mode may reflect the statistical characteristics more accurately. Also, the current block and the neighboring blocks may have the same or similar intra prediction. When the context model for the intra prediction mode information is determined based on the MPM candidate of the MPM list derived based on the neighboring block, the characteristics of the intra prediction mode for the current block are considered. The intra prediction mode information may be coded, and thus, the coding efficiency of the intra prediction mode information may be improved.

A method of setting another context model according to the intra prediction mode will be described later as in the following embodiments.

For example, the intra prediction mode may be divided into four groups, and each group may have a different context model. For example, the intra prediction mode may be divided into a planner intra prediction mode, a DC intra prediction mode, an intra prediction mode with a horizontal direction, and an intra prediction mode with a vertical direction, and the planner intra prediction mode may be a context model 0, a DC intra mode. The prediction mode may have a context model 1, the intra prediction mode having a horizontal direction may have a context model 2, and the intra prediction mode having a vertical direction may have a context model 3. For example, the intra prediction mode having the horizontal directionality may be an intra prediction mode of Nos. 2 to 34, and the intra prediction mode having the vertical directionality may be an intra prediction mode of Nos. 35 to 66. For example, a context map representing a context model for the intra prediction mode may be represented as the following table.

Thus, for example, the context model for the intra prediction mode information may be derived based on the intra prediction mode indicated by the MPM candidate included in the MPM list for the current block. When the intra prediction mode indicated by the MPM candidate is a planar intra prediction mode, the context model for the intra prediction mode information may be derived as the context model 0, and the intra prediction mode indicated by the MPM candidate Is a DC intra prediction mode, the context model for the intra prediction mode information may be derived as context model 1, and if the intra prediction mode indicated by the MPM candidate is an intra prediction mode having a horizontal direction, the intra The context model for prediction mode information may be derived from context model 2, and when the intra prediction mode indicated by the MPM candidate is an intra prediction mode having vertical direction, the context model for the intra prediction mode information may be context. Can be derived from model 3. Here, for example, the intra prediction mode information may indicate the MPM index.

In another example, the intra prediction mode may be divided into three groups, and each group may have a different context model. For example, the intra prediction mode may be divided into planner intra prediction mode, DC intra prediction mode, and directional intra prediction mode, planner intra prediction mode is context model 0, DC intra prediction mode is context model 1, directional intra prediction mode. May have a context model2. Here, for example, the directional intra prediction mode may indicate an intra prediction mode 2 to 66. For example, a context map representing a context model for the intra prediction mode may be represented as the following table.

Thus, for example, the context model for the intra prediction mode information may be derived based on the intra prediction mode indicated by the MPM candidate included in the MPM list for the current block. When the intra prediction mode indicated by the MPM candidate is a planar intra prediction mode, the context model for the intra prediction mode information may be derived as the context model 0, and the intra prediction mode indicated by the MPM candidate Is the DC intra prediction mode, the context model for the intra prediction mode information may be derived from the context model 1, and if the intra prediction mode indicated by the MPM candidate is the directional intra prediction mode, the intra prediction mode information The context model for may be derived from the context model 2. Here, for example, the intra prediction mode information may indicate the MPM index.

In another example, the intra prediction mode may be divided into seven groups, and each group may have a different context model. For example, the intra prediction mode may be divided into a planner intra prediction mode, a DC intra prediction mode, and a plurality of directional subgroups, wherein the planner intra prediction mode is context model 0, the DC intra prediction mode is context model 1, and the first The directional subgroup may have a context model 2, the second directional subgroup has a context model 3, the third directional subgroup has a context model 4, the fourth directional subgroup has a context model 5, and the fifth directional subgroup has a context model 6 have.

Here, for example, the first directional subgroup may include intra prediction modes 2-11, and the second directional subgroup may include intra prediction modes 12-12. The three-directional subgroup may include intra prediction modes 25 to 43, the fourth directional subgroup may include intra prediction modes 44 to 56, and the fifth directional subgroup may include 57 to intra prediction modes. 66 may include an intra prediction mode. In addition, unlike the above-described example, the plurality of directional subgroups may be set. The context map representing the context model for the intra prediction mode may be represented as the following table.

Thus, for example, the context model for the intra prediction mode information may be derived based on the intra prediction mode indicated by the MPM candidate included in the MPM list for the current block. When the intra prediction mode indicated by the MPM candidate is a planar intra prediction mode, the context model for the intra prediction mode information may be derived as the context model 0, and the intra prediction mode indicated by the MPM candidate When the DC intra prediction mode, the context model for the intra prediction mode information can be derived as the context model 1, when the intra prediction mode represented by the MPM candidate is the intra prediction mode of the first directional subgroup, The context model for the intra prediction mode information may be derived from context model 2, and when the intra prediction mode indicated by the MPM candidate is an intra prediction mode of a second directional subgroup, the context model for the intra prediction mode information may be obtained. The context model can be derived from context model 3 And when the intra prediction mode indicated by the MPM candidate is an intra prediction mode of a third directional subgroup, the context model for the intra prediction mode information may be derived as a context model 4, and the MPM candidate indicates When the intra prediction mode is the intra prediction mode of the fourth directional subgroup, the context model for the intra prediction mode information may be derived as context model 5, and the intra prediction mode indicated by the MPM candidate is a fifth directional sub. In the case of intra prediction mode of a group, the context model for the intra prediction mode information may be derived as context model 6. Here, for example, the intra prediction mode information may indicate the MPM index.

Meanwhile, as described above, although the context model for the intra prediction mode information may be derived based on the intra prediction mode indicated by the MPM candidate of the MPM list for the current block, the intra prediction mode and the MPM indicated by the MPM candidate may be derived. A context model for the intra prediction mode information may be derived based on the position of the candidate in the MPM list. The position in the MPM list of the MPM candidates may be indicated by the order in the MPM list. A method of deriving a context model for the intra prediction mode information based on the intra prediction mode indicated by the MPM candidate and the order in the MPM list of the MPM candidate may be the same as described below.

For example, a context map representing a context model according to the order of the intra prediction mode and the MPM list of the MPM candidate may be represented as the following table.

For example, the context model for the intra prediction mode information may be derived based on the intra prediction mode indicated by the MPM candidate included in the MPM list for the current block and the order in the MPM list of the MPM candidate. Twelve context models for the intra prediction mode information may be applied. When the intra prediction mode indicated by the MPM candidate is a DC intra prediction mode and the MPM candidate is an MPM candidate in the first order of the MPM list, the context model for the intra prediction mode information may be derived as the context model 0. And the intra prediction mode indicated by the MPM candidate is a planner intra prediction mode, and when the MPM candidate is an MPM candidate in the first order of the MPM list, the context model for the intra prediction mode information is the context model 1. And the context for the intra prediction mode information when the intra prediction mode indicated by the MPM candidate is an intra prediction mode having horizontal directionality and the MPM candidate is an MPM candidate in the first order of the MPM list The model can be derived from the context model 2, When the intra prediction mode indicated by the MPM candidate is an intra prediction mode having vertical directionality and the MPM candidate is an MPM candidate in the first order of the MPM list, the context model for the intra prediction mode information is referred to as the context model 3. Can be derived. Here, for example, the intra prediction mode information may be an MPM index, and the derived context model may be a context model for the first bin of the intra prediction mode information.

Further, when the intra prediction mode indicated by the MPM candidate is a DC intra prediction mode and the MPM candidate is the MPM candidate of the second order of the MPM list, the context model for the intra prediction mode information is referred to as the context model 4. And if the intra prediction mode indicated by the MPM candidate is a planner intra prediction mode, and the MPM candidate is an MPM candidate in the second order of the MPM list, the context model for the intra prediction mode information is the context. Model 5, wherein the intra prediction mode indicated by the MPM candidate is an intra prediction mode having horizontal directionality, and when the MPM candidate is the MPM candidate of the second order of the MPM list, The context model can be derived from the context model 6 When the intra prediction mode indicated by the MPM candidate is an intra prediction mode having vertical directionality and the MPM candidate is an MPM candidate in the second order of the MPM list, the context model for the intra prediction mode information may include the context model. Can be derived as 7. Here, for example, the intra prediction mode information may be an MPM index, and the derived context model may be a context model for a second bin of the intra prediction mode information.

In addition, when the intra prediction mode indicated by the MPM candidate is a DC intra prediction mode and the MPM candidate is an MPM candidate in the third order of the MPM list, the context model for the intra prediction mode information is referred to as the context model 8. When the intra prediction mode indicated by the MPM candidate is a planner intra prediction mode, and the MPM candidate is an MPM candidate in the third order of the MPM list, the context model for the intra prediction mode information is determined by the context. Model 9, wherein the intra prediction mode indicated by the MPM candidate is an intra prediction mode having horizontal directionality, and when the MPM candidate is the MPM candidate in the third order of the MPM list, The context model may be derived from the context model 10. When the intra prediction mode indicated by the MPM candidate is an intra prediction mode having vertical directionality and the MPM candidate is an MPM candidate in the third order of the MPM list, the context model for the intra prediction mode information is the context model. 11 can be derived. Here, for example, the intra prediction mode information may be an MPM index, and the derived context model may be a context model for a third bin of the intra prediction mode information.

5 schematically illustrates an image encoding method by an encoding apparatus according to the present invention. The method disclosed in FIG. 5 may be performed by the encoding apparatus disclosed in FIG. 1. Specifically, for example, S500 to S520 of FIG. 5 may be performed by the prediction unit of the encoding apparatus, and S530 to S540 may be performed by the entropy encoding unit of the encoding apparatus. In addition, although not shown, a process of deriving a residual sample for the current block based on an original sample and a prediction sample for the current block may be performed by a subtractor of the encoding apparatus. The generating of the information about the residual on the current block may be performed by a converter of the encoding apparatus, and the encoding of the information about the residual may be performed by an entropy encoding unit of the encoding apparatus. It can be performed by.

The encoding apparatus configures a Most Probable Mode (MPM) list of the current block based on the neighboring blocks of the current block (S500). Here, as an example, the MPM list may include three MPM candidates, five MPM candidates, or six MPM candidates.

For example, the encoding apparatus may construct the MPM list of the current block based on the neighboring block of the current block, and the MPM list may include six MPM candidates. The neighboring block may include a pre-left peripheral block, the upper peripheral block, the lower left peripheral block, the upper right peripheral block, and / or the upper left peripheral block of the current block. The encoding apparatus may search the neighboring blocks of the current block in a specific order, and may derive the intra prediction mode of the neighboring block as the MPM candidate in the derived order. For example, an encoding apparatus may include an intra prediction mode of the left neighboring block, an intra prediction mode of the upper neighboring block, a planner intra prediction mode, a DC intra prediction mode, an intra prediction mode of the lower left neighboring block, and the right upper neighboring block. Intra prediction mode of, the intra prediction mode of the upper left neighboring block may be searched in order to derive an MPM candidate and configure the MPM list of the current block. Meanwhile, if six MPM candidates are not derived after the search, an MPM candidate may be derived based on an intra prediction mode derived as the MPM candidate. For example, when the intra prediction mode derived as the MPM candidate is N intra prediction mode, the encoding apparatus sets the N + 1 intra prediction mode and / or N-1 intra prediction mode to the MPM candidate of the current block. Can be derived.

The encoding apparatus selects one of the MPM candidates of the MPM list, and determines the selected MPM candidate as an intra prediction mode for the current block (S510). The encoding apparatus may perform various intra prediction modes to derive an intra prediction mode having an optimal RD cost as an intra prediction mode for the current block. The intra prediction mode may be one of two non-directional intra prediction modes and 65 intra directional prediction modes. As described above, the two non-directional intra prediction modes may include an intra DC mode and an intra planner mode.

In addition, the encoding apparatus may select an MPM candidate having an optimal RD cost among the MPM candidates of the MPM list, and determine the selected MPM candidate as an intra prediction mode for the current block.

The encoding apparatus generates a prediction sample for the current block based on the intra prediction mode (S520). The encoding apparatus may derive at least one neighboring sample of the neighboring samples of the current block based on the intra prediction mode, and generate the predictive sample based on the neighboring sample. The peripheral samples may include upper left corner peripheral samples, upper peripheral samples, and left peripheral samples of the current block. For example, when the size of the current block is WxH and the x component of the top-left sample position of the current block is 0 and the y component is 0, the left neighboring samples are p [-1] [0. ] To p [-1] [2H-1], the sample around the upper left corner is p [-1] [-1], and the sample around the upper side is p [0] [-1] to p [2W-1] [-1].

The encoding apparatus generates at least one bin for intra prediction mode information indicating the selected MPM candidate (S530). The encoding apparatus may generate a bin indicating the value of the intra prediction mode information indicating the index value of the selected MPM candidate. When the MPM list consists of six candidates, the intra prediction mode information may consist of one to five bins. In addition, the value of the intra prediction mode information may be a binary value binarized through a truncated binary code. Here, the intra prediction mode information may indicate an MPM index indicating the selected MPM candidate among MPM candidates of the MPM list. The MPM index may be signaled in the form of an mpm_idx or intra_luma_mpm_idx syntax element.

The encoding apparatus derives a context model for the Nth bin for the intra prediction mode information based on the Nth MPM candidate included in the MPM list (S540). Here, N may be 3 or less.

For example, the context model for the first bin in the intra prediction mode information for the current block may be derived based on the first candidate included in the MPM list. In addition, when the value of the intra prediction mode information for the current block is the second bin is generated, the context model for the second bin may be derived based on the second candidate included in the MPM list. In addition, when a third bin for intra prediction mode information on the current block is generated, the context model for the third bin may be derived based on a third candidate included in the MPM list.

The context model for the Nth bin derived based on the Nth candidate may be as follows.

For example, the context model for the Nth bin may be derived based on an intra prediction mode indicated by the Nth MPM candidate.

For example, when the intra prediction mode indicated by the Nth MPM candidate is a planar intra prediction mode, the context model for the Nth bin may be derived as context model 0, and the Nth MPM candidate is When the intra prediction mode represented is a DC intra prediction mode, the context model for the Nth bin may be derived as context model 1, and the intra prediction mode represented by the Nth MPM candidate is an intra prediction having horizontal direction. In the case of the mode, the context model for the Nth bin may be derived from the context model 2, and if the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode having vertical direction, The context model can be derived from context model 3. The intra prediction mode having the horizontal directionality may be an intra prediction mode of Nos. 2 to 34, and the intra prediction mode having the vertical directionality may be an intra prediction mode of Nos. 35 to 66.

As another example, when the intra prediction mode indicated by the Nth MPM candidate is a planar intra prediction mode, the context model for the Nth bin may be derived as the context model 0, and the Nth When the intra prediction mode indicated by the MPM candidate is a DC intra prediction mode, the context model for the Nth bin may be derived as context model 1, and the intra prediction mode represented by the Nth MPM candidate is directional intra prediction. In the case of the mode, the context model for the Nth bin may be derived from the context model 2. In this case, the directional intra prediction mode may indicate an intra prediction mode from 2 to 66.

As another example, when the intra prediction mode indicated by the Nth MPM candidate is a planar intra prediction mode, the context model for the Nth bin may be derived as the context model 0, and the Nth When the intra prediction mode indicated by the MPM candidate is a DC intra prediction mode, the context model for the Nth bin may be derived as context model 1, and the intra prediction mode represented by the Nth MPM candidate is first directional. In the intra prediction mode of the subgroup, the context model for the Nth bin may be derived from context model 2, and the intra prediction mode indicated by the Nth MPM candidate is the intra prediction mode of the second directional subgroup. In this case, the context model for the Nth bin may be derived as context model 3, and the Nth If the intra prediction mode indicated by the MPM candidate is the intra prediction mode of the third directional subgroup, the context model for the Nth bin may be derived as context model 4, and the intra prediction indicated by the Nth MPM candidate If the mode is the intra prediction mode of the fourth directional subgroup, the context model for the Nth bin may be derived as context model 5, and the intra prediction mode indicated by the Nth MPM candidate is a fifth directional subgroup. In the intra prediction mode of, the context model for the Nth bin may be derived as the context model 6. Here, the first directional subgroup may include intra prediction modes 2 to 11, and the second directional subgroup may include intra prediction modes 12 to 24, and the third directional subgroup May include intra prediction modes 25 to 43, and the fourth directional subgroup may include intra prediction modes 44 to 56, and the fifth directional subgroup may include intra predictions 57 to 66. It may include a mode.

In another example, the context model for the Nth bin may be derived based on an intra prediction mode indicated by the Nth MPM candidate and an order (ie, N) in the MPM list of the Nth MPM candidate. have. That is, even when the first MPM candidate and the second MPM candidate having different order in the MPM list indicate the same intra prediction mode, the context model of the bean for the first MPM candidate and the context of the bean for the second MPM candidate The model can be derived differently.

For example, when the intra prediction mode indicated by the Nth MPM candidate is a DC intra prediction mode and N is 1, the context model for the Nth bin may be derived as context model 0, and the N If the intra prediction mode indicated by the first MPM candidate is a DC intra prediction mode and N is 2, the context model for the Nth bin may be derived as context model 4, and the Nth MPM candidate indicates the When the intra prediction mode is the DC intra prediction mode and N is 3, the context model for the Nth bin may be derived as context model 8.

Further, for example, when the intra prediction mode indicated by the Nth MPM candidate is a planner intra prediction mode and N is 1, the context model for the Nth bin may be derived as context model 1, When the intra prediction mode indicated by the Nth MPM candidate is a planner intra prediction mode and N is 2, the context model for the Nth bin may be derived as context model 5, and the Nth MPM candidate is When the intra prediction mode represented is a planar intra prediction mode and N is 3, the context model for the Nth bin may be derived as context model 9.

Also, for example, when the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode having horizontal directionality and N is 1, the context model for the Nth bin is derived from context model 2. If the intra prediction mode indicated by the N-th MPM candidate is an intra prediction mode having horizontal directionality, and N is 2, the context model for the N-th bin may be derived as a context model 6, When the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode having horizontal directionality and N is 3, the context model for the Nth bin may be derived as a context model 10.

Further, for example, when the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode having vertical directionality and N is 1, the context model for the Nth bin is derived from context model 3. If the intra prediction mode represented by the N-th MPM candidate is an intra prediction mode having vertical directionality, and N is 2, the context model for the N-th bin may be derived as a context model 7, When the intra prediction mode indicated by the N th MPM candidate is an intra prediction mode having vertical direction, and N is 3, the context model for the N th bin may be derived as a context model 11.

The encoding device encodes the Nth bin for the intra prediction mode information based on the context model (S550). For example, the encoding apparatus may perform arithmetic encoding on the Nth bin based on the context model. The encoding apparatus may generate the encoded intra prediction mode information by performing arithmetic encoding on the Nth bin of the intra prediction mode information based on the context model. Meanwhile, when the fourth bin for the intra prediction mode information is generated, the encoding apparatus may perform bypass encoding on the fourth bin. In addition, when a fifth bin for the intra prediction mode information is generated, the decoding apparatus may perform bypass encoding on the fifth bin. In this case, the bypass encoding may represent a method of encoding by applying a uniform probability distribution (for example, 50:50) instead of applying a context model having a specific probability distribution.

The encoding apparatus may output image information including the encoded intra prediction mode information in the form of a bitstream.

Meanwhile, the encoding apparatus may generate an MPM flag indicating whether the determined intra prediction mode is included in the MPM candidates of the MPM list. The image information may include the MPM flag. When the determined intra prediction mode is included in the MPM candidates of the MPM list, the intra prediction mode information indicating the determined intra prediction mode among the MPM candidates may be encoded. On the other hand, when the determined intra prediction mode is not included in the MPM candidates of the MPM list, remaining intra prediction mode information indicating the determined intra prediction mode among remaining intra prediction modes not included in the MPM candidates. Can be generated. Meanwhile, when the determined intra prediction mode is included in the MPM candidates of the MPM list, the encoding apparatus may not signal the MPM flag, and the value of the MPM flag may be derived as 1.

Meanwhile, the bitstream may be transmitted to a decoding device through a network or a (digital) storage medium. The network may include a broadcasting network and / or a communication network, and the digital storage medium may include various storage media such as USB, SD, CD, DVD, Blu-ray, HDD, SSD, and the like.

6 schematically illustrates an encoding apparatus for performing an image encoding method according to the present invention. The method disclosed in FIG. 5 may be performed by the encoding apparatus disclosed in FIG. 6. Specifically, for example, the prediction unit of the encoding apparatus of FIG. 6 may perform S500 to S520 of FIG. 5, and the entropy encoding unit of the encoding apparatus of FIG. 6 may perform S530 to S550 of FIG. 5. Although not shown, a process of deriving a residual sample for the current block based on the original sample and the prediction sample for the current block may be performed by the subtraction unit of the encoding apparatus of FIG. 6. The generating of the information about the residual for the current block based on the residual sample may be performed by the transform unit of the encoding apparatus of FIG. 6, and the encoding of the residual information may be performed in FIG. 6. May be performed by an entropy encoding unit of the encoding apparatus.

7 schematically illustrates an image decoding method by a decoding apparatus according to the present invention. The method disclosed in FIG. 7 may be performed by the decoding apparatus disclosed in FIG. 2. Specifically, for example, S710 to S720 of FIG. 7 may be performed by the entropy decoding unit of the decoding apparatus, and S700 and S730 to S740 may be performed by the prediction unit of the decoding apparatus. In addition, although not shown, a process of obtaining image information including information about a residual and / or intra prediction mode information about the current block through a bitstream may be performed by an entropy decoding unit of the decoding apparatus. The derivation of the residual sample for the current block based on the residual information may be performed by an inverse transform unit of the decoding apparatus, and the reconstructed picture is based on the prediction sample and the residual sample. The generating process may be performed by an adder of the decoding apparatus.

The decoding apparatus configures a Most Probable Mode (MPM) list of the current block based on the neighboring blocks of the current block (S700). Here, as an example, the MPM list may include three MPM candidates, five MPM candidates, or six MPM candidates.

For example, the decoding apparatus may configure the MPM list of the current block based on the neighboring block of the current block, and the MPM list may include six MPM candidates. The peripheral block may include the left peripheral block, the upper peripheral block, the lower left peripheral block, the upper right peripheral block, and / or the upper left peripheral block of the current block. The decoding apparatus may search the neighboring blocks of the current block in a specific order, and may derive the intra prediction mode of the neighboring block as the MPM candidate in the derived order. For example, a decoding apparatus may include an intra prediction mode of the left neighboring block, an intra prediction mode of the upper neighboring block, a planner intra prediction mode, a DC intra prediction mode, an intra prediction mode of the lower left neighboring block, and the right upper neighboring block. Intra prediction mode of, the intra prediction mode of the upper left neighboring block may be searched in order to derive an MPM candidate and configure the MPM list of the current block. Meanwhile, if six MPM candidates are not derived after the search, an MPM candidate may be derived based on an intra prediction mode derived as the MPM candidate. For example, when the intra prediction mode derived as the MPM candidate is N intra prediction mode, the decoding apparatus sets the N + 1 intra prediction mode and / or the N-1 intra prediction mode to the MPM candidate of the current block. Can be derived.

Meanwhile, the decoding apparatus may obtain image information including intra prediction mode information about the current block from the bitstream. In this case, the intra prediction mode information may indicate an MPM index indicating one of the MPM candidates of the MPM list. The MPM index may be signaled in the form of an mpm_idx or intra_luma_mpm_idx syntax element. When the MPM list is composed of six candidates, the value of the intra prediction mode information may be represented by a binary value consisting of one to five bins.

Also, for example, the image information may include an MPM flag for the current block. When the value of the MPM flag is 1, the decoding apparatus may obtain the intra prediction mode information for the current block from the bitstream. Alternatively, the video information may not include the MPM flag. In this case, the decoding apparatus may derive the value of the MPM flag as 1.

In addition, when the value of the MPM flag is 0, the decoding apparatus may obtain the remaining intra prediction mode information for the current block from the bitstream. That is, when the value of the MPM flag is 0, the image information may include remaining intra prediction mode information indicating one of the remaining intra prediction modes. In this case, the decoding apparatus may derive the intra prediction mode indicated by the remaining intra prediction mode information among the remaining intra prediction modes as the intra prediction mode for the current block. Here, the remaining intra prediction modes may indicate remaining intra prediction modes not included in the MPM candidates of the MPM list. The remaining intra prediction mode information may be signaled in the form of rem_intra_luma_pred_mode or intra_luma_mpm_remainder syntax elements.

The decoding apparatus derives a context model for the Nth bin for the intra prediction mode information based on the Nth MPM candidate included in the MPM list (S710). Here, N may be 3 or less.

For example, the context model for the first bin in the intra prediction mode information for the current block may be derived based on the first candidate included in the MPM list. In addition, the context model for the second bin may be derived based on the second candidate included in the MPM list, and the context model for the third bin may be derived based on the third candidate included in the MPM list.

The decoding apparatus decodes the Nth bin based on the context model to derive the value of the intra prediction mode information (S720). For example, the decoding apparatus may perform arithmetic decoding on the Nth bin based on the derived context model. The decoding apparatus may perform arithmetic decoding of the N-th bin by predicting the occurrence probability of the N-th bin of the intra prediction mode information based on the context model, and may determine the value of the intra prediction mode information. The corresponding symbol can be generated.

On the other hand, when the value of the intra prediction mode information includes a fourth bin, the decoding apparatus may perform bypass decoding on the fourth bin. In addition, when the value of the intra prediction mode information includes a fifth bin, the decoding apparatus may perform bypass decoding on the fifth bin. In this case, the bypass decoding may indicate a method of decoding by applying a uniform probability distribution (for example, 50:50) instead of applying a context model having a specific probability distribution.

The decoding apparatus derives the MPM candidate having the same index value as the value of the intra prediction mode information among the MPM candidates of the MPM list in the intra prediction mode for the current block (S730). The decoding apparatus may derive the MPM candidate indicated by the value of the intra prediction mode information among the MPM candidates of the MPM list as the intra prediction mode for the current block. That is, the decoding apparatus may derive the MPM candidate having the same index value as the value of the intra prediction mode information among the MPM candidates as the intra prediction mode for the current block.

The decoding apparatus generates a prediction sample for the current block based on the intra prediction mode (S740). The decoding apparatus may derive at least one neighboring sample of the neighboring samples of the current block based on the intra prediction mode, and generate the predictive sample based on the neighboring sample. The peripheral samples may include upper left corner peripheral samples, upper peripheral samples, and left peripheral samples of the current block. For example, when the size of the current block is WxH and the x component of the top-left sample position of the current block is 0 and the y component is 0, the left neighboring samples are p [-1] [0. ] To p [-1] [2H-1], the sample around the upper left corner is p [-1] [-1], and the sample around the upper side is p [0] [-1] to p [2W-1] [-1].

Although not shown in the drawing, the decoding apparatus may directly use the prediction sample as a reconstruction sample according to a prediction mode, or generate a reconstruction sample by adding a residual sample to the prediction sample. If there is a residual sample for the current block, the decoding apparatus may receive information about the residual for the current block, and the information about the residual may be included in the information about the face. The information about the residual may include transform coefficients regarding the residual sample. The decoding apparatus may derive the residual sample (or residual sample array) for the current block based on the residual information. The decoding apparatus may generate a reconstructed sample based on the prediction sample and the residual sample, and may derive a reconstructed block or a reconstructed picture based on the reconstructed sample. Thereafter, as described above, the decoding apparatus may apply an in-loop filtering procedure, such as a deblocking filtering and / or SAO procedure, to the reconstructed picture in order to improve subjective / objective picture quality as necessary.

8 schematically illustrates a decoding apparatus for performing an image decoding method according to the present invention. The method disclosed in FIG. 7 may be performed by the decoding apparatus disclosed in FIG. 8. Specifically, for example, the entropy decoding unit of the decoding apparatus of FIG. 8 may perform S710 to S720 of FIG. 7, and the prediction unit of the decoding apparatus of FIG. 8 may perform S700 and S730 to S740 of FIG. 7. . In addition, although not shown, the process of acquiring image information including information on the residual of the current block and / or intra prediction mode information through the bitstream may be performed by the entropy decoding unit of the decoding apparatus of FIG. 8. The derivation of the residual sample for the current block based on the residual information may be performed by an inverse transform unit of the decoding apparatus of FIG. 8, wherein the prediction sample and the residual sample are performed. The process of generating the reconstructed picture may be performed by the adder of the decoding apparatus of FIG. 8.

According to the present invention described above, it is possible to derive a context model for coding a bin for intra prediction mode information of the current block in consideration of the MPM candidate of the MPM list of the current block. It is possible to reduce the amount of bits to represent and improve the overall coding efficiency.

According to the present invention, a context model for coding a bin for intra prediction mode information of the current block is considered in consideration of the intra prediction mode indicated by the MPM candidate of the MPM list of the current block and the order in the MPM list of the MPM candidate. In this case, it is possible to reduce the amount of bits for indicating the intra prediction mode of the current block and to improve the overall coding efficiency.

In the above-described embodiment, the methods are described based on a flowchart as a series of steps or blocks, but the present invention is not limited to the order of steps, and any steps may occur in a different order or simultaneously from other steps as described above. have. In addition, those skilled in the art will appreciate that the steps shown in the flowcharts are not exclusive and that other steps may be included or one or more steps in the flowcharts may be deleted without affecting the scope of the present invention.

The above-described method according to the present invention may be implemented in software, and the encoding device and / or the decoding device according to the present invention may perform image processing of, for example, a TV, a computer, a smartphone, a set-top box, a display device, and the like. It can be included in the device.

When embodiments of the present invention are implemented in software, the above-described method may be implemented as a module (process, function, etc.) for performing the above-described function. The module may be stored in memory and executed by a processor. The memory may be internal or external to the processor and may be coupled to the processor by various well known means. The processor may include application-specific integrated circuits (ASICs), other chipsets, logic circuits, and / or data processing devices. The memory may include read-only memory (ROM), random access memory (RAM), flash memory, memory card, storage medium and / or other storage device. That is, the embodiments described in the present invention may be implemented and performed on a processor, a microprocessor, a controller, or a chip. For example, the functional units shown in each drawing may be implemented and performed on a computer, processor, microprocessor, controller, or chip.

In addition, the decoding apparatus and encoding apparatus to which the present invention is applied include a multimedia broadcasting transmitting and receiving device, a mobile communication terminal, a home cinema video device, a digital cinema video device, a surveillance camera, a video chat device, a real time communication device such as video communication, and mobile streaming. Devices, storage media, camcorders, video on demand (VoD) service providers, over the top video (OTT) devices, internet streaming service providers, 3D (3D) video devices, video telephony video devices, and medical video devices It may be included and used to process a video signal or a data signal. For example, the OTT video device may include a game console, a Blu-ray player, an internet access TV, a home theater system, a smartphone, a tablet PC, a digital video recorder (DVR), and the like.

In addition, the processing method to which the present invention is applied can be produced in the form of a program executed by a computer, and can be stored in a computer-readable recording medium. Multimedia data having a data structure according to the present invention can also be stored in a computer-readable recording medium. The computer readable recording medium includes all kinds of storage devices and distributed storage devices in which computer readable data is stored. The computer-readable recording medium may be, for example, a Blu-ray disc (BD), a universal serial bus (USB), a ROM, a PROM, an EPROM, an EEPROM, a RAM, a CD-ROM, a magnetic tape, a floppy disk, and an optical disc. It may include a data storage device. The computer-readable recording medium also includes media embodied in the form of a carrier wave (eg, transmission over the Internet). In addition, the bitstream generated by the encoding method may be stored in a computer-readable recording medium or transmitted through a wired or wireless communication network. In addition, an embodiment of the present invention may be implemented as a computer program product by program code, which may be performed on a computer by an embodiment of the present invention. The program code may be stored on a carrier readable by a computer.

In addition, the content streaming system to which the present invention is applied may largely include an encoding server, a streaming server, a web server, a media storage, a user device, and a multimedia input device.

The encoding server compresses content input from multimedia input devices such as a smart phone, a camera, a camcorder, etc. into digital data to generate a bitstream and transmit the bitstream to the streaming server. As another example, when multimedia input devices such as smart phones, cameras, camcorders, etc. directly generate a bitstream, the encoding server may be omitted. The bitstream may be generated by an encoding method or a bitstream generation method to which the present invention is applied, and the streaming server may temporarily store the bitstream in the process of transmitting or receiving the bitstream.

The streaming server transmits the multimedia data to the user device based on the user's request through the web server, and the web server serves as a medium for informing the user of what service. When a user requests a desired service from the web server, the web server delivers it to a streaming server, and the streaming server transmits multimedia data to the user. In this case, the content streaming system may include a separate control server. In this case, the control server plays a role of controlling a command / response between devices in the content streaming system.

The streaming server may receive content from a media store and / or an encoding server. For example, when the content is received from the encoding server, the content may be received in real time. In this case, in order to provide a smooth streaming service, the streaming server may store the bitstream for a predetermined time.

Examples of the user device include a mobile phone, a smart phone, a laptop computer, a digital broadcasting terminal, a personal digital assistant (PDA), a portable multimedia player (PMP), navigation, a slate PC, Tablet PCs, ultrabooks, wearable devices, such as smartwatches, glass glasses, head mounted displays, digital TVs, desktops Computer, digital signage, and the like. Each server in the content streaming system may be operated as a distributed server, in which case data received from each server may be distributed.

Claims

In the image decoding method performed by the decoding apparatus,

Constructing a Most Probable Mode (MPM) list of the current block based on neighboring blocks of the current block;

Deriving a context model for an Nth bin for intra prediction mode information based on an Nth MPM candidate included in the MPM list, wherein N is 3 or less;

Decoding the Nth bin based on the context model to derive the value of the intra prediction mode information;

Deriving an MPM candidate having an index value equal to that of the intra prediction mode information among the MPM candidates of the MPM list, into an intra prediction mode for the current block; And

And generating a predictive sample for the current block based on the intra prediction mode.
The method of claim 1,

And the context model for the Nth bin is derived based on an intra prediction mode indicated by the Nth MPM candidate.
The method of claim 2,

If the intra prediction mode indicated by the Nth MPM candidate is a planar intra prediction mode, the context model for the Nth bin is derived as context model 0,

When the intra prediction mode indicated by the Nth MPM candidate is a DC intra prediction mode, the context model for the Nth bin is derived as context model 1,

When the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode having horizontal directionality, the context model for the Nth bin is derived as context model 2,

And if the intra prediction mode indicated by the N th MPM candidate is an intra prediction mode having vertical direction, the context model for the N th bin is derived as context model 3.
The method of claim 3,

The intra prediction mode having horizontal directionality is an intra prediction mode of 2 to 34, and the intra prediction mode having vertical directionality is an intra prediction mode of 35 to 66.
The method of claim 2,

When the intra prediction mode indicated by the Nth MPM candidate is a planar intra prediction mode, the context model for the Nth bin is derived as the context model 0,

When the intra prediction mode indicated by the Nth MPM candidate is a DC intra prediction mode, the context model for the Nth bin is derived as context model 1,

And if the intra prediction mode indicated by the Nth MPM candidate is a directional intra prediction mode, the context model for the Nth bin is derived as context model 2.
The method of claim 2,

When the intra prediction mode indicated by the Nth MPM candidate is a planar intra prediction mode, the context model for the Nth bin is derived as the context model 0,

When the intra prediction mode indicated by the Nth MPM candidate is a DC intra prediction mode, the context model for the Nth bin is derived as context model 1,

If the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode of the first directional subgroup, the context model for the Nth bin is derived as context model 2,

If the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode of a second directional subgroup, the context model for the Nth bin is derived as context model 3,

If the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode of a third directional subgroup, the context model for the Nth bin is derived as context model 4,

If the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode of a fourth directional subgroup, the context model for the Nth bin is derived as context model 5,

And if the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode of a fifth directional subgroup, the context model for the Nth bin is derived as a context model 6.
The method of claim 6,

The first directional subgroup includes intra prediction modes 2 to 11, the second directional subgroup includes intra prediction modes 12 to 24, and the third directional subgroup includes 25 to 43 times. And an intra prediction mode, wherein the fourth directional subgroup includes intra prediction modes 44 through 56, and the fifth directional sub group includes intra prediction modes 57 through 66. Way.
The method of claim 1,

And the context model for the Nth bin is derived based on the intra prediction mode indicated by the Nth MPM candidate and the order in the MPM list of the Nth MPM candidate.
The method of claim 8,

If the intra prediction mode indicated by the Nth MPM candidate is a DC intra prediction mode and N is 1, the context model for the Nth bin is derived as context model 0,

If the intra prediction mode indicated by the Nth MPM candidate is a DC intra prediction mode and N is 2, the context model for the Nth bin is derived as context model 4,

And if the intra prediction mode indicated by the Nth MPM candidate is a DC intra prediction mode and N is 3, the context model for the Nth bin is derived as a context model 8.
The method of claim 8,

If the intra prediction mode indicated by the Nth MPM candidate is a planner intra prediction mode and N is 1, the context model for the Nth bin is derived as context model 1,

If the intra prediction mode indicated by the Nth MPM candidate is a planner intra prediction mode and N is 2, the context model for the Nth bin is derived as context model 5,

And if the intra prediction mode indicated by the Nth MPM candidate is a planar intra prediction mode and N is 3, the context model for the Nth bin is derived as context model 9.
The method of claim 8,

When the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode having horizontal directionality and N is 1, the context model for the Nth bin is derived as context model 2,

When the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode having horizontal directionality and N is 2, the context model for the Nth bin is derived as context model 6,

When the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode having horizontal directionality and N is 3, the context model for the Nth bin is derived as a context model 10. Way.
The method of claim 8,

If the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode having vertical direction, and N is 1, the context model for the Nth bin is derived as context model 3,

If the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode having vertical orientation, and N is 2, the context model for the Nth bin is derived as a context model 7,

When the intra prediction mode indicated by the Nth MPM candidate is an intra prediction mode having vertical directionality and N is 3, the context model for the Nth bin is derived as a context model 11. Way.
In the video encoding method performed by the encoding device,

Constructing a Most Probable Mode (MPM) list of the current block based on neighboring blocks of the current block;

Selecting one of the MPM candidates in the MPM list and determining the selected MPM candidate as an intra prediction mode for the current block;

Generating a prediction sample for the current block based on the intra prediction mode;

Generating at least one bin for intra prediction mode information indicating the selected MPM candidate;

Deriving a context model for an Nth bin for the intra prediction mode information based on an Nth MPM candidate included in the MPM list, wherein N is 3 or less; And

And encoding the N th bin for the intra prediction mode information based on the context model.
The method of claim 12,

And the context model for the N th bin is derived based on an intra prediction mode indicated by the N th MPM candidate.
The method of claim 12,

And the context model for the Nth bin is derived based on the intra prediction mode indicated by the Nth MPM candidate and the order in the MPM list of the Nth MPM candidate.